Evidence for Internally Generated Magnetic Fields on the Cv Chondrite Parent Planetesimal

Introduction: The aggregational texture and bulk chemical composition of chondrites demonstrate that they are not the products of planetary melting proceses. As a result, chondrites have traditionally been thought to samples bodies that have not undergone large-scale differentiation [1]. However, it has long been known that CV and other carbonaceous chondrites carry a stable natural remanent magnetization [24]. The origin of this magnetization has been a longstanding mystery because it was in many cases acquired following accretion of the parent body [4, 5]. The spinning, orbiting body would have been unlikely to record the spatially and temporally variable magnetic fields associated with the T Tauri sun and the protoplanetary disk [6]. On the other hand, the possibility of an internally generated dynamo field [5] has been discounted due to the assumption that chondrites are samples of undifferentiated bodies [6]. We have conducted a paleomagnetic study on the Allende CV3 carbonaceous chondrite. Our goal is to determine the nature and acquisition time for the magnetization in Allende in order to distinguish between core dynamo and external field sources. Measurements: We conducted alternating field and thermal demagnetization, rock magnetic, and paleointensity measurements on 51 mutually oriented subsamples of Allende sample AMNH 5056 (Fig. 1). Most of our subsamples contain a mixture of matrix, chondrules and/or refractory inclusions, although several consisted of nearly pure CAIs. Interior samples: Samples from the interior (> 1 mm from fusion crust) have natural remanent magnetization (NRM) composed of at least two components: a low coercivity (LC) component up to ~10 mT and an extremely high coercivity (HC) component from ~10 to > 290 mT (Fig. 2). The HC magnetization is uniformly oriented throughout the meteorite’s interior (Fig. 2). ARM and IRM paleointensities (following [7]) indicate that the paleointensity of the magnetizing field was of order 10 μT (for comparison, Earth’s present surface field strength is ~50 μT). Similar results were obtained by [8, 9]. Our thermal demagnetization analyses demonstrate the presence of a stable magnetic component ranging from ~320-500°C and confirm results from previous investigators [6, 10] that most of the NRM is carried by pyrrhotite and other sulfides, with a small contribution from magnetite and possibly metal. Fig. 1. Photograph of Allende sample AMNH 5056 showing our sampling locations (polygons) and corresponding subsample numbers. Blue = 18 fusioncrusted subsamples, white = 33 interior subsamples. Samples are all mutually oriented.